Secondary batteries presently in wide use include lead batteries and nickel-cadmium batteries wherein the single-cell voltage is about 2 V, and an aqueous solution is used. In recent years, efforts have been made to investigate and develop secondary batteries of high energy density which give a high single-cell voltage of at least 3 V and include a negative electrode of lithium. However when lithium is used it reacts with water or the and aprotic electrolytes must be used since aqueous electrolytes not usable. Although polar organic solvents are presently wide use, a majority of these solvents have a low boiling point (high vapor pressure) are inflammable and therefore involve the likelihood of staining neighboring members and ignition or firing due to a leak or break and tile hazard of explosion due to erroneous use or overcharging. Furthermore, repeated discharge and charge of the secondary battery as contemplated form dendrites on the negative electrode, entailing the problem of reduced discharge-charge efficiency and short-circuiting between the positive and negative electrodes. Accordingly, many reports have been made on the development of techniques for improving the discharge-charge efficiency of the negative electrode and the cycle life by inhibiting dendrite formation. Proposed in these reports are, for example, use of a methylated cyclic ether solvent as the solvent for battery electrolytes (K. H. Abraham et al. in "Lithium Batteries", J. P. Gabano, editor, Academic Press, London (1983)), a method of forming an ionically conductive protective film at the Li interface by adding polyethylene glycol, polypropylene glycol, polyethylene oxide or like additive to an electrolyte system ( Journal of Power Sources, Vol 12, No. 2, pp. 83-144 (1984) and Unexamined Japanese Patent Publication S H 0 60-41773), a method of inhibiting Li dendrite formation by alloying an electrode per se with Al (Unexamined Japanese Patent Publication S H 0 59-108281).
On the other hand, M. Armand and N. Duelot disclose a novel secondary battery of high energy density incorporating a thin-film polymer electrolyte in Laid-Open French Patent Publication No. 2442512 and European Patent No. 13199. Yao et al. (J. I norg. Nucl. Chem., 1967, 29, 2453) and Farrington et al. (Science, 1979, 204, 1371) generally describe inorganic ionically conductive solids. These solids, which are powdery, must be pelletized by a high-pressure press for fabrication into batteries. This offers a great obstacle against productivity, uniformity, etc. The pelletized solid is hard and brittle, is therefore difficult to make into a thin film of increased area, and requires a great pressure when it is to be adhered to the active electrode substance, that the procedure has problems in work efficiency and adhesion. Furthermore, the solid encounters difficulty in following and compensating for variations in the volume of electrode materials during the operation of the battery and has the hazard of breaking the electrolyte. Sequlir et al. ( Extended Abstracts, 163rd Meeting Electrochemical Society, 1983, 83, 751, Abstract, No. 493) describe a battery of novel design including a solvent-free thin-film polymer electrolyte, stating that the electrolyte is usable at a medium temperature of about 100.degree. C. as determined by testing. However, the conductivity at room temperature is as low as 10.sup.-6 .about.10.sup.-7 S/cm and is insufficient.
P. M. Blonsky et al. ( J. Am. Chem. Soc., 106, 6854, 1984) state that di{2-(2-methoxyethoxy)ethoxy}polyphosphazene (M E E P ) is useful as an electrolyte or electrochemical batteries. However, they merely disclose data as to a.c. conductivity in the range of from 30.degree. C. to 97.degree. C. and have not effected discharge and charge with d.c.
As stated above, the problems encountered in developing high-performance secondary batteries are the formation of dendrites, leakage and hazards. The object of development is to complete high-performance secondary batteries which are free of these drawbacks, assured of safety, operable at usual ambient temperatures and at least 3 V in single-cell voltage.
An object of the present invention is to provide a lithium secondary battery comprising two electrode active substances which have a high, energy density, and an electrolyte which is flame-retardant, nonflowable and safe and has such high ion conductivity that it maintains its characteristics even at low ambient temperatures of not higher than the freezing point of water.